Differential transformer



June 2, 1953 w. D. MACGEORGE DIFFERENTIAL TRANSFORMER Original Filed Nov. 24, 1948 INVENTOR Wzzzzam a/l/laaaawye.

JIM/2 4. (BWQM' ATTORNEY- Patented June 2, 1953 UNI-TED STATES PATENT OFFICE.

BIFFEEENTIAL TRANSFORMER William DPMacgeorge, Collegeville, Pa., assignor to Automatic Temperature Control 00., Inc;, Philadelphia; Pa a corporation of P'ennsyb- Vania 1 Claim.. (Cl. 336-436) t This invention" relates" to differential transformers and'constitutes'a division of application Ber; No; 61,885; filed November 24; 1948, which issued'as- Patent No. 2568587, onseptember 1'8; 1951-.

Differential transformers have been" used to a considerable degree for various control functions ihvarious' typesof circuits; but so far as known each'has comprised a single primary winding and a: pair of secondary windings, or reversals of these elements in various arrangements thereof. An illustrative circuit and differential transformer assembly is'shown; for instance, in my Patent'No: 24513757. A specific differential transformer of the prior art" is also" shown in my earlier Patent No. 2,4273%. In" this latter patent three axially aligned coils are provided, of which the endcoils are secondaries coupled in bucking" opposition and disposed symmetrically: on both: sides of" the central primary coil. An armature" mass is-m'ounted-for axial relative adjustment on an axis concentric With the cm mon axis of the" thrce'coils. When the primary is energizedby A. C. the output of the coupled secondariesis a resultant of the combined theoretically-cancelling outputs of the respective secondaries: Theoretically; with the armature in a centered position; the voltages induced in the"respectivesecondaries are equal and of opposite and cancellingphases; so that the resultant output is null. As the armature and i the aligned coils are relativelyshilted' in position" ineither direction from the null position, the'volt'age'increasesinone secondary and decreases'i'n-the other secondary; so that there is a result'ant output of a phase ofthe preponderating volt'agegand in amplitude a function of the movement away from the null position. Actual experience with difierential transformers ofthis type indic'ates certain definite disadvantages and inaccuracies of this type of differential tranc former, which militate against the accuracy and sensitivity thereof. For instance, thefl'uxpath o'fthe'transformer is relatively short with a consequent restriction on the length of the linear portion of" the output curve thereof; The relative spacings between the secondaries and the primaryis very critical, andeven a minute differencebetween the two respective spaces changes theiuilloutput and may alsoeive an'output curve-on the respective sides of the nullpos-itime, which is asymmetrical relativeto the null output. The windings have relatively high impedance with a poor capacity distribution so that" evenat the theoretical null position there is always a strong-residual output, ofthe order perhaps of as-much. as a. volt; on: evenhighen, frornthe secondaries dependinguponv the: ex:- citing" frequency used. This is: because; the capacity: effects prevent. the full: reversal; of: the phase of. the respective voltages of the" second; aries; Obviously this reduces: the numbers of stages of amplification that can: be: associated with the transformer. A further disadvane tageous function of such; differential; trans? formers develops 1 when two or: more: are associated in a commoncircuit. Inzmy: circuitxpatent above mentioned; there is' disclosed one: three? coil differential. transformer;. the armature of which is moved i in response. to a change; of: cone dition, to furnish". an output: as a function: of; the displacement". of the armature: The outputis in a circuitzwitli a motor: and with. aw. second: dittferenti'al transformer of the sameitype, therposition of the armature of which latterris-z adjusted as a function of motor: running; and: whenxthc output of the second transformer: equaisand. C3111.- cels the outputofthe filsttransformer the motor circuit becomes null and the: motor stops. Manufacturing tolerancesv for: the: formers: upon which such three-coll transformers are: wound arenot small QHOUghLiIllPlTQCtiCB to secure: identis cal spacings between the: primary andzt'he sec;- ondaries, andl evenr a minute: difference between the coils shifts the: null. When: two: transformers are associated in' the: same circuit the traveling null; i; e. that point at which: the: circuitishalanced, whether. at an. actuallsupposedlnull relative" position of the respective'armatunesiin their transformers, or: at any other IBltt'biV8\ position at whichthe outputs of. the transformers are equal and cancelling, becomes by the inherent properties of the transformers; indefinte and inaccurate. This causes: errors to: enter into} the indications or controlling; functions accomplished by the. motor" of: the" circuit, because the actual motor running to accomplish operative circuit null ipositions inayl'he short, or, on the other, hand, an overrunnine: of the true stoppage. point. While in many inst'allationsthese errors, which may berelatively small',lmay= be permitted within the tolerances allowed; in many other installations where precision and. exactness aneprerequisites, the systems; have not. proven-r satisfactory. At best such transformers haverequired skill andtiine andadditional:items of equipment in endeavoring to balance or compensate for-the errors in the" instant transformers Finally" the differential transformers of. the prior arthhave been=offixed and inflexible characteristics having; one, invariable tum ratio of primary turns; to: secondary turns; incapable of variations or predeterminations to accord with the requirements of specific installations, and additionally have been of considerable size and of appreciable cost.

It is among the objects of this invention to provide a differential transformer which obviates the disadvantages of the prior art; to provide a differential transformer which is smaller and more compact, and is also cheaper and far more accurate than has heretofore been attained; to provide a differential transformer which incorporates both the actuating or exciting primary and the two coupled or couplable secondaries in an entity for the axial reception and movement of an armature, while being only of two-coil length; to provide improved circuits incorporating the improved transformer whereby selective results can be attained which were previously not readily attainable by the prior art differential transformers; to provide coils for dif ferential transformers arranged for intercom nection in a complete differential transformer of predetermined but selectively variable characteristics; to provide a differential transformer of predetermined selected ratios of output voltages to input voltages; to provide differential transformers of two identical double wire coils having end connections and arranged for interconnection so that selectively the primaries can be coupled in parallelism 01' in series bucking or aiding relation and in which the secondaries can be selectively coupled in parallelism, or in series aiding or bucking relation to provide selected characteristics of the complete transformer; to improve inductance bridges; to combine an inductance bridge with a differential transformer to effect improved circuit control systems; to provide primary and secondary windings as simultaneously formed turns in adjacency in two spaced but contiguous coil units so as to effect exact output and exciting coil arrangements to insure exact matching of related coils; to effect improvements in the details of differential transformers; to provide a differential transformer of relatively low impedance; to provide a differential transformer of even capacity distribution; to provide a differential transformer with an elongated flux path; to provide a two-coil differential transformer with the primaries coextensive with the secondaries in both respective coils as to secure such evenly distributed capacity that the null output when armature is properly associated therewith is of such small amplitude as compared with prior art three-coil trans formers as to be susceptible to appreciably greater amplification than such three-coil trancformers; to provide a differential transformer as two composite coils each comprising identi cal parallel turns of primary and secondary windings, with the coils in axial alignment with the coils spaced at an optimum but not critical spacing, wherein the inductive and capacitive effects of the respective coils are identical and evenly dispersed throughout both coils; to improve circuits containing differential transformers; and to provide other objects and advantages as will become more apparent as the description proceeds.

In the accompanying drawings forming part of this invention:

Fig. 1 represents a longitudinal diametric section through the complete transformer according to this invention, without the wiring connections thereof being disclosed in detail, and showing the relatively movable armature in its concentric axially movable position therein, and each coil of the transformer being comprised of plural adjacent turns of separate wires so that the primary and secondary of each coil of the two aligned coils are in adjacency throughout the turns of each coil.

Fig. 2 represents a diagram of the wiring of the transformer in an illustrative coupling for a bridge circuit arrangement.

Referring to Fig. l, the transformer of this invention is shown as comprising illustratively a cylindrical former element l 0, having the axial bore I I extending for the full length thereof, and having an outer winding surface formed as two aligned surfaces respectively l2 and I3, separated from each other by the median division or radial wall I4, and terminating respectively in end walls or radial annular divisions I5 and IE. The hollow winding former element thus described is formed of suitable non-inductive ma terial such as a ceramic or phenolic condensation product or the like. It will be observed that the external surfaces of the former provide re spectively a winding space [1, defined by walls l4 and I5 and peripheral surface l2, and a winding space [8 defined by the walls I4 and I6 and peripheral surface l3. The axial bore II is of such size as to accommodate and permit relative axial motion of an armature mass 20, under the actuation of a control link or extension 2| generally aligned with the bore.

In the simplest form of the invention and as shown in Fig. 2, a primary winding 22, indicated in light lines comprising a first wire, and a secondary winding 23, indicated by the heavier lines comprising a second wire respectively in said diagram as one composite coil 24 of the transformer, and a primary winding 25 indicated by light lines comprising a third wire, and a secondary winding 26 indicated by the full lines in said diagram, comprising a fourth wire, as the other composite coil 27, are provided. The coils 24 and 21 are wound as follows: Wires 22 and 23 are laid together at one end of the space H, and wires 25 and 26 are laid together at one end of the space I 8, with predetermined lengths of ends thereof extending generally radially or axially of the former for ultimate interconnection with other ends to be described. Maintaining equal tensions on both wires of both coils, the former is then rotated and the pairs of wires are thus laid on and guided by complementa-lly moving traversing guides or the like back and forth to effect a tight winding of both wires simultaneously in lateral and vertical adjacency for the desired number of turns and multiple layers of turns in both coils. Free ends of the last turns are exposed for ultimate interconnection according to the desired results, and the given transformer, basically has then been formed. It will be observed that there are exactly equal turns of both primaries and secondaries in both coils on the same diameters so that the initial inductive effects of both coils are identical and with the primary coextensive with the secondary in both coils and capacity is evenly distributed throughout each coil. The conclusion of the final winding step finds two relatively disconnected slightly spaced coil masses of generally cylindrical form. in axialspacingbutinaxialalignment. Thefreeends of both coil masses may pass inwardly or outwardly of the former and of the coils, or the ends may pass axially through contiguous walls for ultimate interconnection as will be described. It will be clear that the former may comprise 87.. removable arbor; and the: walls maybe replaced'byairgaps; andthe support'of the coils maybe: by'an" external sleeve; or by'a skeleton o1" tubular internal' guide-or they may be securedtogethermerelyby suitable adhesive, or molded together'ina' binder, if desired, without departing from" the spirit of the invention.-

The" method of forming the transformer outlined ab'ove conducestoready and facile changes in our preselection of the interconnection of the terminals ofthe coils for the attainment of pre" determined characteristics of the ultimate transformer.

Inproducing desired differential transformers accordingtolthis invention, it isnecessary that the respective four windings of the respectivetwo composite coils be considered as two primaries and two secondaries. A source of A. C. as the means for energizing the primaries, as indicated at 29, and an instrument 1.! is shown which is to be understood as either an instrument or the input to a further complete circuit modified or affected by the transformers shown. For simplicity in the diagrams, single turns are illus-- trated, but it will be understood that there are multiple layers of multiple turns in each composite coil.

Pig. 2 represents the utilization of the winch ings described in both a voltage bridge circuit and an inductive bridge circuit. While these are combined in the differential transformer organization disclosed, comprising a dual transmitter feeding two measuring circuits, it will be seen that either circuit can be established to function as a single bridge circuit.

According to this figure the differential transformer secondaries comprise two legs of a voltage bridge circuit, the other half of which is formed by a voltage divider, and the output of which is controlled by the relative positioning of the armature 20 of the differential transformer, and the armature 2!) of the inductively controlled voltage divider. Similarly, the primaries of the differential transformer power the volt age bridge and also comprise two legs of an inductive bridge circuit, the other half of which is comprised of a voltage divider, and the output of which is controlled by the relative positioning of the armature 2B of the differential transformer and the positioning of the armature 20 of the primary circuit inductively controlled voltage divider.

The voltage bridge is comprised of the secondaries 2.3 and 26 which are in series aiding relation, as are the primaries 22 and 25. However, the adjacent ends of the secondaries have a common. return path by a lead 30. The free ends of the secondaries are looped. with the respective ends of the voltage divider comprised of coils 3! and 32 to form the voltage bridg and the divider ill completes, with lead the bridge output circuit. With armature 26 positioned symmetrically of the interconnection of the secondaries 23 and .26 at St, so that equal voltages are induced in the secondaries, and with the armature 2i! relatedly positioned relative to the coils 3i and 32, the secondary output on one side of the interconnection in one secondary will be equal to and in phase with the voltage in the secondary on the other side of the interconnection. Displacement of the armature 2!! will unbalance the voltages in the legs of the voltage bridge and cause a bridge output signal, which is neutralized or nullified by adjustments of the armature 20 in the proper sense or vice versa.

This" isbecause movement-*- of the armature toward one-or the othersecondarycoil will increasethe voltage-output of that coil and decrease that of theother as a linear function of suclrmovement'.

Theinductance bridgeis similarly formed by the-provision of a-common return lead 35 forming the interconnectionbetween primary coils 22 and 25, and" thefree ends of these coils are joinedin-a-bridgecircuit with a voltage divider comprisedof coillegs-33 and 34; the divider 36 of whichcompletes, with lead 35; the output of theinductance bridge, whichiscontrolledby the relative positioning of the armature 29 and as, as willhe understood:

In this caseit will be'clear that more'windings than the two described could housed in connec tion with both i coils;

InFig; Zdt will be seen that with the primaries energized oy the source of A. 0., the armature 26, coupled, for instance, to the movable element of a condition-responsive element, such, for instance, as that which will respond to temperature, pressure, or the like, is arranged for axial shifting along the common axis of the two composite coils of the transformer. At some point at which the center of the mass is symmetrically between the two composite coils there will be developed a substantially null output from the bridge, manifested as an indication in the instrument is or in the amplifying circuit with which it is associated. There will be at the null point a residual output of a small voltage of the order of one or more millivolts, but this is relatively inconsequential and if the instrument It is replaced by connections as the input of a control circuit, any desired degree of amplification can be used with safety and propriety so as to secure the desired control results. Movement of the armature in either direction induces a resultant bridge output which increases from null as a linear function of armature movement with the length of the linear portion of the curve appreciably longer than is possible of the three-coil transformer of the prior art. By the transformer of this invention the length of the flux path of the transformer is increased, the spacing between coils is not critical, and the linear portion of the output curve of the transformer greatly lengthas compared to the prior art three-coil assemblies. There is optimum spacing of the order of one-quarter inch between the composite coils of the instant invention, but this is not critical, as slight deviations either way may occur without appreciable change in efficiency.

It will be further understood that within the purview of the invention it is not necessary that the number of windings or of the separate wires used in a compound or composite winding be restricted to the two already described, as the invention has utility when three or even more windinge are simultaneously applied to make up the complete composite coil.

Having thus described my invention, I claim:

A transformer comprising a first and a second air -cored coil in axial alignment and juxtaposed relation in a nonunagnetic assembly, the first coil being formed of at least a first and a second wire, and the second coil being formed of at least tl: and a fourth wire, of which said first and th. identical and said second and fourth are and fourth Wires respectively in mutual inductive relation throughout the operative lengths of said wire in their respective coils, means interconnecting said first and third Wires to form a transformer primary having connections for an energizing A. C. input, means interconnecting said second and fourth Wires to form a transformer secondary connectively isolated from the transformer primary comprising a common output connection for the second and fourth wires, said second and fourth wires having respective terminal output connections each respectively similarly spaced from and defining with the common connection substantially identical efiective operative lengths of said second and fourth wires, a magnetic armature movable axially of the two aligned coils having one substantially symmetrical position therein at which the voltage of the efiective operative length of said second Wire is of substantially the same magnitude and phase as that of the eflective operative length of said fourth wire, movement of said armature from said symmetrical position in either sense increasing the magnitude of the voltage in the operative length of that wire of the secondary toward which the armature moves while reciprocally decreasing that of the other wire of the secondary as a linear function of such movement.

WILLIAM D. MACGEORGE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,568,587 Macgeorge Sept. 18, 1951 2,569,105 James Sept. 25, 1951 2,570,672 Hathaway Oct. 9, 1951 

